The present invention generally relates to chemical mechanical planarization, and more particularly to a method and apparatus for determining the endpoint of a planarization process.
Chemical mechanical planarization (CMP) is now common in the manufacturing process of semiconductors. Semiconductors are constructed of multiple layers of material deposited on a wafer. CMP may be used throughout the manufacturing process of semiconductors to planarize the various layers of material deposited on the wafer or to remove the various layers entirely. For example, CMP may be used to remove a layer of metal, such as copper, that previously had been deposited on the wafer to form interconnects.
CMP is generally accomplished by pressing a surface of the wafer against a polishing pad attached to a rotating or orbiting platen in the presence of slurry. During the planarization process, it is desirable to gather data on the condition of the wafer""s surface. The data may then be used to optimize the planarization process or to determine when the planarization process should be terminated (referred to as xe2x80x9cendpointxe2x80x9d), for example, when a metal layer has been substantially removed from the wafer. Typically, such data is collected by endpoint detection (EPD) systems.
Many users prefer EPD systems that are xe2x80x9cin-situxe2x80x9d EPD systems, which provide EPD during the polishing process. One group of prior art EPD systems for detecting the endpoint during the CMP of thin films involves optical systems that use interferometric or spectrophotometric optical measurement techniques. Using these techniques, a light is directed to the wafer""s surface. A reflected light is collected and the interferometric, spectrophotometric and/or other changes in the reflected light is measured to determine the endpoint. However, in CMP processes where the material removal rate is slow, the interferometric or spectrophotometric change may be slow and minimal. For example, certain low down-force copper CMP processes are designed to use slow material removal rates to achieve higher levels of surface planarization. Because of the slow removal rates, it is difficult using standard optical EPD techniques to detect the minor interferometric or spectrophotometric changes and, as a result, process endpoints are incorrectly detected.
Accordingly, there exists a need for an optical endpoint detection method and apparatus that accurately detect the endpoint of chemical mechanical planarization of thin films. A further need exists for an optical endpoint detection method and apparatus that accurately detect the endpoint of chemical mechanical planarization processes using slow removal rates.
This summary of invention section is intended to introduce the reader to aspects of the invention and is not a complete description of the invention. Particular aspects of the invention are pointed out in other sections hereinbelow, and the invention is set forth in the appended claims which alone demarcate its scope.
In accordance with an exemplary embodiment of the present invention, a method for detecting the removal of a first layer of material overlying a workpiece during a planarization process is provided. Relative motion is effected between the first layer and a working surface to remove the first layer. A light having a spectrum of wavelengths is transmitted to the workpiece. The intensity for each of a plurality of reflected wavelengths of a reflected light reflected from the workpiece is measured to obtain a spectrum. The spectrum is a function of the plurality of reflective wavelengths. A spectrum reference value is then calculated. A plurality of spectrum difference values are calculated by subtracting the spectrum reference value from the spectrum. An absolute value for each of the plurality of spectrum difference values that is a function of one of the plurality of reflected wavelengths that falls within a selected range of wavelengths is summed together to obtain a delta value. An endpoint parameter is calculated from the delta value.
In accordance with another exemplary embodiment of the present invention, an apparatus for detecting the removal of a first layer of material overlying a workpiece during a chemical mechanical planarization process is also provided. The apparatus includes a light-illuminating assembly for transmitting light to the workpiece during one of a plurality of scan cycles. A light-receiving assembly receives a reflected light reflected from the workpiece during the scan cycle. A processor is operatively connected to the light-receiving assembly and is configured to measure an intensity for each of a plurality of reflected wavelengths of the reflected light to obtain a spectrum. The spectrum is a function of the plurality of reflected wavelengths. The processor is also configured to calculate a reference spectrum value and calculate a plurality of spectrum. Each of the calculated spectrum difference values is a function of one of the plurality of reflected wavelengths. The processor is further configured to sum together absolute values of each of the plurality of spectrum difference values that is a function of one of the plurality of reflected wavelengths that falls within a selected range of wavelengths to obtain a delta value therefrom. The processor is programmed to calculate an endpoint parameter from the delta value.
These and other aspects of the present invention are described in the following description, claims and appended drawings.